Comparison of O2 and NO2 impact on PtOx and PdOx formation in diesel oxidation catalysts and their reduction by CO and C3H6 pulses

2018 
Abstract NO oxidation on diesel oxidation catalyst (DOC) increases NO 2 /NO ratio in the exhaust gas, which is beneficial for both NO x reduction and soot oxidation processes in the catalysts and filters located downstream, such as SCR, LNT and DPF. The impact of O 2 and NO 2 on the PtO x and PdO x formation and subsequent changes in NO oxidation activity was studied on Pt/γ-Αl 2 O 3 and Pd/γ-Αl 2 O 3 catalysts. Repeated heat-up and cool-down temperature ramps in the range of 80–450 °C and isothermal deactivation/reactivation experiments at 150, 175 and 200 °C with NO oxidation as a probe reaction were performed. Inverse hysteresis of NO 2 yield during temperature ramps was observed with both catalysts. Though NO 2 is stronger oxidizing agent than O 2 , PtO x and PdO x formation was induced to a similar extent by both O 2 and NO 2 when present at their typical concentration levels (8% O 2 , 250 ppm NO 2 ). The NO oxidation activity of both Pt and Pd sites was restored by CO or C 3 H 6 pulses while keeping overall lean conditions (excess of oxygen). The maximum efficiency of CO pulses was achieved at 200 °C for PtO x reduction, and at 150 °C for PdO x reduction.
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